The present invention relates to a method and apparatus for plasma processing such as dry etching, sputtering, and plasma CVD which are used in the fabrication of semiconductors or other electronic devices. In particular, the invention relates to a method and apparatus for plasma processing which makes use of low-electronic-temperature plasma.
To keep up with the miniaturization of electronic devices such as semiconductors, it has been discussed in Unexamined Japanese Laid-Open Patent Publication No. 8-83696 that using high-density plasma is important. Meanwhile, in recent years, attention has been focused on low-electronic-temperature plasma of high electron deficiency and low electron temperature.
When gases having a high electronegativity such as Cl.sub.2 and SF.sub.6, i.e., gases that are liable to cause negative ions, are transformed into plasma, electron temperatures of about 3 eV or lower will cause larger amounts of negative ions to be generated as compared with cases of higher electron temperatures. Taking advantage of this phenomenon makes it possible to prevent the configurational abnormality of etching so called notches, which are caused by positive charges accumulating at the bottom of a fine pattern due to excessive incoming positive ions. Thus, etching of extremely fine patterns can be carried out with high precision.
Also, when gases containing carbon and fluorine such as CxFy and CxHyFz (where x, y and z are natural integers), which are commonly used in the etching of silicon oxide film and other insulating films, are transformed into plasma, electron temperatures of about 3 eV or lower will suppress the decomposition of gases, particularly the generation of F atoms, F radicals and the like, as compared with cases of higher electron temperatures. Since F atoms, F radicals and the like, have higher etching speeds on silicon, lower electron temperatures enable an insulating-film etching having a larger selection ratio to silicon to be achieved.
Further, 3 eV or lower electron temperatures will cause the ion temperature to be lower as well, enabling a reduction of ion damage to the substrate in plasma CVD.
In the examples shown above, greater results are generated with lower electron temperatures, where remarkable results could not be expected immediately when the electron temperature has reached 3 eV. Plasma of 2 eV or lower electron temperatures are considered to be one effective for several generations of devices in the future.
In turn, the electron temperature is as high as 4 to 6 eV in ECRP (Electronic Cyclotron Resonance Plasma) or HWP (Helicon Wave Plasma) using a static magnetic field, and 3 to 4 eV in ICP (Inductively Coupled Plasma) using no static magnetic field. The electron temperature for plasma is the most difficult to control among the plasma parameters. It could be said that the electron temperature almost completely depends on the plasma source, i.e. the method of plasma generation. Even if external parameters such as the type of gas, gas flow rate, gas pressure, magnitude of applied high-frequency power and configuration of vacuum chamber are changed, the electron temperature shows almost no changes. However, there have been some methods proposed recently. Some of them are described in detail below.
FIG. 18 is a cross-sectional view of an ICP etching equipment. Referring to FIG. 18, while an interior of a vacuum chamber 21 is held at a specified pressure by simultaneously effecting exhaustion with a pump 23 and introduction of specified gas from a gas supply unit 22 into the vacuum chamber 21, high-frequency power of 13.56 MHz is supplied by a high-frequency power source 24 to a coil 26 placed on a dielectric 25 and grounded at its one end, by which plasma is generated in the vacuum chamber 21, enabling plasma processing such as etching, deposition and surface reforming to be achieved on a substrate 28 placed on an electrode 27. In this process, as shown in FIG. 18, also supplying high-frequency power to the electrode 27 by a high-frequency power source 29 makes it possible to control the ion energy that reaches the substrate 28. In addition, for impedance matching, a matching circuit 30 is disposed between the high-frequency power source 24 and the coil 26. It is known that after the high-frequency power applied to the coil 26 is turned off, the electron temperature rapidly lowers at a time constant on the order of several .mu.sec in the afterglow plasma. Meanwhile, the time constant at which the plasma density decreases is greater than the time constant for the relaxation time of electron temperature, so that modulating the high-frequency power by using a pulse of about 50 to 200 kHz allows the electron temperature to be set to 2 eV or lower without greatly decreasing the electron density. In addition, a technique that differs in coil form but is essentially the same as the foregoing pulse-modulation ICP system is described in detail in J. H. Hahm et al., "Characteristics of Stabilized Pulsed Plasma Via Suppression of Side Band Modes", Proceedings of Symposium on Dry Process (1996). Also, the pulsed discharge plasma and the afterglow plasma are described in detail in Sinriki Teii, "Basic Engineering of Plasma", p. 58, UCHIDA ROKAKUHO PUBLISHING CO., LTD. (1986).
FIG. 19 is a cross-sectional view of an etching equipment on which a spoke antenna type plasma source is mounted. Referring to FIG. 19, while interior of a vacuum chamber 31 is held at a specified pressure by simultaneously effecting exhaustion with a pump 33 and introduction of specified gas from a gas supply unit 32 into the vacuum chamber 31, high-frequency power of 500 MHz is supplied by a high-frequency power source 34 to a spoke antenna 36 placed on a dielectric 35, by which plasma is generated in the vacuum chamber 31, enabling plasma processing such as etching, deposition and surface reforming to be achieved on a substrate 38 placed on an electrode 37. In this process, as shown in FIG. 19, also supplying high-frequency power to the electrode 37 with a high-frequency power source 39 makes it possible to control the ion energy that reaches the substrate 38. For impedance matching, a stub 40 is disposed between the high-frequency power source 34 and the spoke antenna 36. Although clear reasons have not yet been found, a low electron temperature of 2 eV or lower has been realized with a spoke antenna type plasma source using high-frequency power of 500 MHz. In addition, this system is described in detail in S. Samukawa et al., "New Ultra-High-Frequency Plasma Source for Large-Scale Etching Processes", Jpn. J. Appl. Phys., Vol. 34, Pt. 1, no 12B (1995).
However, with the conventional system as shown in FIG. 18, there is an issue with a reflected-wave power of as much as 10% or more of the traveling wave power being generated. This is because a Q (Quality Factor: reactance component of impedance/resistance component), as the range of from the matching circuit 30 to the coil 26 is taken as one load is very high as a result of a narrow-band load, such that matching cannot be obtained for frequency components equal to or other than the fundamental harmonic wave (13.56 MHz) that are generated with pulse modulation and most of the frequency components are returned to the power source as reflected waves. Also, even if plasma is generated under these conditions, the reflected-wave power is not constant at all times, making it very difficult to obtain reproducibility of processing results of the processing rate.
Also, with the conventional system as shown in FIG. 19, there is an issue that plasma is not generated with low pressure. In particular, it is very difficult to generate plasma in low pressure regions of 3 Pa or lower. This is a common issue for plasma sources using frequencies of the UHF band or higher (300 MHz or higher) using no static magnetic field. For example, without the static magnetic field, plasma could not be generated at low pressure even with an ECR plasma source using 2.45 GHz. Since actual plasma processing, such as etching, is usually carried out at around 1 Pa, this system needs to first generate plasma in a high pressure region where plasma generation is ensured, and then changing the pressure to a desired one by increasing the discharge speed of the pump or decreasing the gas flow rate. However, applying such a method would make it impossible to achieve etching or other processing with high precision. To avoid this, it is necessary to generate plasma by generating a strong static magnetic field within the vacuum chamber 1 under control of the pressure to a desired pressure of around 1 Pa, and then enhancing the efficiency of the electron acceleration using magnetic waves, or to generate plasma by using a trigger discharge of some other system. However, using the static magnetic field or the trigger discharge leads to a considerably increased risk of thin insulating-film breakdown in the semiconductor device, called charge-up damage. Further, where a use of frequencies of the UHF band or higher (300 MHz or higher) including 500 MHz would require the stub 40 for impedance matching, it is inevitable that both weight and volume are enlarged as compared with the matching circuit formed up from variable capacitors. This leads to another issue in terms of cost.